1. Field of the Invention
The present invention relates generally to surface acoustic wave signal processors and more particularly to surface acoustic wave signal processors demonstrating substantial insensitivity to variations of the ambient temperature.
2. Description of the Prior Art
Typical of prior art acoustic signal processors are reflective array devices often used in constructing band pass filters, pulse compression filters, and the like. Such devices are normally constructed at the planar surface of a piezoelectric substrate at which are formed opposed input and output interdigital metal film transducers. One or more reflecting arrays are disposed obliquely in the propagation path between the input and output transducers. Each reflective array contains a grouping of parallel reflector elements in the form of metallic surface strips or of grooves etched into the surface of the piezoelectric substrate, for example.
Typical of such prior art devices are processors in which the input transducer launches a wave along the z-axis of the substrate crystal and a single reflective array reflects the acoustic waves so as to propagate energy along the x-axis toward the output transducer. Devices of this type are described in the U.S. Pat. No. 4,055,820 to L. P. Solie for a "Reflective Dot Array for Acoustic Wave Processing", issued Oct. 25, 1977 and assigned to Sperry Corporation. Variations of such devices are also well known in which a pair of arrays is disposed so as effectively to reverse the original direction of the launched wave through 180 angular degrees.
The operating characteristics of these and other such acoustic signal processors are adversely affected by changes in the ambient temperature. So that the necessity of placing of the substrate in a temperature controlled oven may be relieved, a temperature compensated reflective array was devised, as disclosed in the L. P. Solie U.S. Pat. No. 4,166,228 for a "Temperature Compensated Reflective Array for Surface Acoustic Processing", issued Aug. 28, 1979 and also assigned to Sperry Corporation. This prior art temperature-insensitive, reflective surface wave signal processor contains an even number of pairs of cooperating reflective arrays in which successive pairs of the reflective arrays are disposed so as to rotate the direction of propagation of the acoustic wave through 180 angular degrees in opposite directions and in which the propagation path between the individual reflective arrays in each pair is minimized. In this manner, the acoustic signal reaches the output transducer flowing in the same direction as it did upon leaving the input transducer, as will be further discussed.